Bi2O3 Nanoparticles Decorated Carbon Nanotube: An Effective Nanoelectrode for Enhanced Electrocatalytic 4-Nitrophenol Reduction

4-Nitrophenol (4-NP) is present in most industrial waste water resources as an organic pollutant, and is a highly toxic and environmentally hazardous pollutant. Herein, we report that bismuth oxide (Bi2O3) decorated multi-walled carbon nanotubes (Bi2O3@MWCNTs) are the most prominent electrocatalyst for 4-NP electroreduction in acidic conditions. The electrocatalyst is synthesized by a simple chemical reduction method using ethylene glycol as a capping agent. The synthesized Bi2O3@MWCNTs electrocatalyst has been well-characterized by Fourier-transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy. Bi2O3@MWCNTs have a cubic structure which is confirmed by XRD. TEM imaging reveals Bi2O3 NPs are ~2 nm in size, are grown on MWCNTs and that these nanoparticles are active toward 4-NP electroreduction. The electrochemical studies by cyclic voltammetry measurements show that the Bi2O3@MWCNTs electrocatalyst can sense 4-NP at a very low potential i.e., −0.17 vs. saturated calomel electrode (SCE). Furthermore, electroanalytical parameters like scan rate and concentration dependence were studied with electrochemcial impedance spectroscopy (EIS) and the effect of pH on cathodic current was examined under experimental conditions. The lower limit of detection (LOD) was found to be 0.1 μM for the Bi2O3@MWCNTs nanomaterial and is excellent toward 4-NP. The present study has applications for reducing water pollution and for sorting out related issues.

INTRODUCTION 4-Nitrophenol (4-NP) is an important compound as it is a precursor in the manufacturing processes of drugs, pesticides, dyes, fungicides, insecticides, explosives, and is used to darken leather (Li et al., 2012;Veerakumar et al., 2015;Rajkumar et al., 2018;Wu et al., 2018). However, it is a hazardous compound and has been shown to be present in industrial waste water and in fresh water where it comes from agricultural field run-off due to the degradation of organo-phosphorus pesticides (Wu et al., 2018). 4-NP is responsible for soil and water pollution because of its high level of toxicity. Due to this, the US Environmental Protection Agency (US EPA) has listed 4-NP as a priority pollutant. According to the US EPA the acceptable limit of 4-NP in potable water is GRAPHICAL ABSTRACT | Electrochemical Determination of 4-Nitrophenol on Bi 2 O 3 @MWCNT Nano electrode. only 60 ppb. Consuming even a small amount leads to acute effects in humans such as nausea, headache, drowsiness and cyanosis (Bose and Ramiah, 2017). It is even more dangerous as it can act as carcinogenic and mutagenic agent (Rajkumar et al., 2018). So there is an urgent need for the detection of 4-NP.
In the literature there are various methods employed for the determination of 4-NP viz. Belloli et al. (1999) determined 4-NP levels through a simple isocratic HPLC approach using seven columns, where the columns contained silica based C 18 material. In this process a SiOH cartridge was used, which was treated with methyl chloride, dried with helium gas and stored at a low temperature. Sampling took 4 h. Guo et al. (2004) performed the separation and determination of phenol isomers by using capillary zone electrophoresis with methanol as an additive, it is an efficient method and about 10 5 theoretical plates per meter was achieved. Zhong et al. (2011) reported a method of detection in waste water, it is a 3-step process comprised of sample preparation, determination by GC-MS and subsequent analysis. Lin et al. (2013) proposed spectrophotometric detection, in which 4-NP was pre-concentrated with 1-hydroxy-2-naphthoic acid modified TiO 2 , under optimum conditions more than 98% adsorption can be achieved by this method. Each method has certain limitations associated with it, such as the detection limit, sensitivity, selectivity, heavy instrumentation requirements, use of expensive gases, pretreatment etc. Moreover, the electrochemical method is a prominent technique for the detection of effluents like 4-NP as it is a rapid, cost effective, easy to operate, highly sensitive, and most importantly on-site measurement, that requires mild reaction conditions and has a low limit of detection. These factors make it a better alternative compared to the rest of the methods (Sun et al., 2012;Lin et al., 2013;Barman et al., 2017). Due to these factors, many electrochemical sensors have been reported for the determination of 4-NP. Chemically modified electrodes proved to be the best electrocatalysts as they increase the rate of reaction, provide a high surface area, have good stability and selectivity. A literature survey revealed that graphene based electrocatalysts (Li et al., 2012), metal and metal oxide electrocatalysts (Barman et al., 2017;Singh et al., 2017), metal-free electrocatalysts (Rajkumar et al., 2018), and their composites were all briefly studied as methods to determine 4-NP concentration.
There are several materials used as conductive substrates on which metal nanoparticles are anchored and grown including graphene (Chen et al., 2011), carbon nanotubes (Umar et al., 2016), Fullerene C 60 , graphitic carbon nitride Hassannezhad et al., 2019) and conducting polymers like polyaniline (Wang G et al., 2018). Among these, carbon nanotubes have drawn attention since their inception in 1991, because of their unique properties including excellent electrical conductivity, large surface area, high electron density, exceptionally high mechanical and chemical stability, all of which makes them an ideal supporting material (Umar et al., 2016;Liu et al., 2018). Furthermore, the Ni based nanocomposite, MgFe 2 O 4 NPs proved to be a good electrocatalyst for 4nanoparticles (NPs) reduction (Baby et al., 2019;Mejri et al., 2019). Carbon nanotubes decorated with metals like Pt, Pd, Ag, monometallic-Au, and bimetallic-Au show excellent electrocatalytic activity toward nitrophenols Umar et al., 2016;Sun et al., 2017;Dhanasekaran et al., 2019;Ding et al., 2019), but their low earth abundance and high cost inhibits their practical applicability. In order to address these issues researchers have proposed alternative approaches including replacing the precious metals with non-noble metals or producing metal free systems with high stability and cost effectiveness.
Literature shows that bismuth (Bi) and Bi 2 O 3 films, nanoparticles and hybrids show excellent electrocatalytic activity toward 4-NP (Hutton et al., 2004;Lezi et al., 2014;Xia et al., 2014). This is because Bi and Bi 2 O 3 electrocatalysts shows a high surface area, low band gap, and have good chemical and electrochemical properties. Additionally to this, Bi based systems have been shown to be1 good electrochemical sensors for heavy metals, biomolecules, drugs and for the electrocatalytic reduction of nitrates (Zhang et al., 2009;van der Horst et al., 2016;Sakthivel et al., 2019). These merits of Bi motivate researchers to use it for the electrochemical sensing of 4-NP. Bi proved to be a good alternative for precious metals as it is cost effective, sensitive, environmental friendly, versatile, and has a variety of uses (Hutton et al., 2004;Dortsiou and Kyriacou, 2009;Švancara et al., 2010;Wang W. et al., 2019). A Bi 2 O 3 multiwalled carbon nanotube (Bi2O 3 @MWCNT) composite might provide synergistic effects for the enhanced electrochemical detection of 4-NP. To date, and to the best of our knowledge, Bi 2 O 3 @MWCNTs has not been reported for 4-NP detection and quantification.

EXPERIMENTAL Chemicals
MWCNTs 99.99%, Ethylene glycol (EG) 99.97%, Sulphuric acid 98%, Nitric acid 78%, Bismuth pentahydrate [Bi(NO 3 ) 3 (H 2 O) 5 ], Acetone 99.99% and 4-NP (Para-nitrophenol) were purchased from Alfa-Aesar. All the chemicals were used as received for the synthesis of electrocatalysts and electrochemical studies were carried out in deionized water. Characterization X-ray diffraction (XRD) was carried out using a Rigaku Ultima IV fully automatic high-resolution X-ray diffractometer, with the Xray generator operating at 40 kV and 40 mA at a step of 0.01(2θ) at room temperature. FTIR spectra were measured in the 4,000-400 cm −1 range on a Perkin-Elmer Spectrum-I spectrometer with samples prepared as KBr pellets. Raman spectroscopy was performed using a microscope with Raman optics (Seki Technetronic Corporation, Tokyo) with a 532 nm LASER.

Electrochemical Measurements
All electrochemical studies were performed on a CHI660C electrochemical works station (CH-Instrument) with a three electrode system. A glassy carbon (GC, 3 mm dia.) electrode was used as the working electrode to support the catalysts. A Pt foil and saturated SCE were used as the counter and reference electrodes, respectively. The GC electrode was polished with three different sizes of Al 2 O 3 powder (1, 0.3, and 0.05 µm) followed by cleaning in an ultrasonic bath and finally rinsing with deionized water followed by ethanol. To prepare the working electrode, 0.5 mg as-synthesized catalyst was dispersed in a calculated amount (2-6 mL) of isopropyl alcohol under ultrasonic stirring for 40 min. An aliquot of the slurry was dropped onto the pre-polished GC electrode by using a micropipette and dried under an infrared lamp. The loading of catalyst on the electrode was calculated and used for normalization of the current.

Synthesis of Acid Functionalized Multiwalled Carbon Nanotubes (MWCNTs)
In a typical synthesis, 1 g of MWCNT powder was added to 120 mL nitrating mixture (30 mL HNO 3 and 90 mL H 2 SO 4 ) in a round bottom flask which was placed in an ice bath under stirring for 30 min. Then mixture was then subjected to SCHEME 1 | Oxidative functionalization of MWNTs followed by decoration with Bi 2 O 3 NPs. Frontiers in Chemistry | www.frontiersin.org ultrasonication at room temperature for 2 h. After sonication the dispersion was refluxed for 6 h, and a brown-gray paste was obtained after completion of the reaction (defined as diminished effervesence). After separating it via centrifugation, the browngray paste was re-dispersed in deionized (DI) water and subjected to ultrasonication for 3 h. After filtration the obtained solid material was washed with 1 M HCl, followed by an adequate amount of DI water until it turned into a black powdery material. These MWCNTs were used for further characterization and doping with bismuth.

Synthesis of Bi 2 O 3 Decorated Multiwalled Carbon Nanotubes and Bi 2 O 3 NPs
In a typical synthesis, 50 mL of ethylene glycol (EG) was heated at 110 • C for 30 min under constant stirring to remove dissolved oxygen and water molecules. This obtained anhydrous EG was used for further synthesis. Following this, 20 mg of acid functionalized MWCNTs were added to 50 mL anhydrous EG and dispersed by sonication for 3 h. On complete dispersion of the MWCNTs, bismuth nitrate (0.02 gm in 50 mL EG) was added dropwise under constant stirring and stirring continued for 3 h. Then this reaction mixture was refluxed for 6 h. The resultant product was cooled to RT then filtered and washed with acetone. The obtained black colored catalyst was dried in the oven at 60 • C for 2 h which results in Bi 2 O 3 @MWCNTs

RESULTS AND DISCUSSION
The morphological and structural features of electrocatalysts were identified by TEM and are shown in Figure 1a. The TEM image of the MWCNTs shows the average diameter is ∼10 nm. Moreover, Figure 1b shows the TEM for Bi 2 O 3 @MWCNTs. MWCNTs were found to provide a large surface area for crystalline Bi NPs with spherical shape. Further, the superimposed FTIR spectrum of MWCNTs, Bi 2 O 3 NPs and Bi 2 O 3 @MWCNTs is displayed in Figure 2 and it clearly shows bands appearing at 1,500, 1,700, and 3,600 cm −1 which correspond to the stretching frequencies of oxygen in C=O, aromatic C=C and O-H, respectively, all of which are functional groups present on MWCNTs. Bi 2 O 3 @MWCNTs clearly indicates that the peak around 600 cm −1 and 1,000 cm −1 corresponds to Bi-O stretching frequencies (Chipeture et al., 2019). The XRD (crystal structure) images of MWCNTs and Bi 2 O 3 @MWCNTs is displayed in Figure 3A and shows sharp signals for Bi 2 O 3 NPs and MWCNTs in the 2θ of range 10 to 90 • . The characteristic diffraction patterns (002) and (101) correspond to the MWCNTs, which closely agree with reported functionalized MWCNTs systems. Moreover, Bi 2 O 3 has an α-metastable crystal structure that was confirmed by corresponding peaks observed at the (110), (116), and (300) planes. Raman spectra for MWCNTs and the Bi 2 O 3 @MWCNTs composite are shown in Figure 3B. In case of MWCNTs a signal appeared corresponding to the D band at 1,345 cm −1 and the G band at 1,570 cm −1 and their associated intensity ratio (i.e., I D /I G ) ratio is 0.32. For Bi 2 O 3 @MWCNTs the calculated intensity ratio, I D /I G , is 0.88. The increase of I D /I G by more than double confirms an increase in the disorder in MWCNTs after decoration with Bi 2 O 3 NPs. It is in good agreement with previous reports (Thi Mai Hoa, 2018).
Thermogravimetric analysis deals with weight loss during decomposition as a function of temperature, in the range of 0-1,000 • C. In case of MWCNTs the degree of functionalization, i.e., the extent to which carboxylic groups were grafted onto MWCNTs, was confirmed by TGA. Figure 4 shows the superimposed TGA curves for MWCNTs, Bi 2 O 3 NPs and Bi 2 O 3 @MWCNTs. In the case of MWCNTs (red line) a twostep decomposition has been observed, in the first step, which ranges from 100 to 300 • C, ∼ 23% of weight was lost due to the loss of water molecules and other volatile species. In the second stage, ranging between 300 and 520 • C, rapid weight loss was observed to give an overall weight loss of 58%, this could be due to functional groups like -COOH and is in good agreement with the literature (Thi Mai Hoa, 2018). In the case of Bi 2 O 3 NPs (blue line) a two-step weight loss is observed. The first step between 70 and 270 • C corresponds to 13% weight loss due to the elimination of water and other impurities. While in the second step, after 270 • C, an overall 33 % weight loss is observed. This is due to the low melting point of Bi (271 • C) and the removal of capping molecules. When compared to MWCNTs & Bi 2 O 3 NPs,   the Bi@MWCNTs nanocomposite (black line) shows much more thermal stability. It shows a rapid decomposition after 271 • C which is attributed to the low melting point of bismuth.

Electrochemical Studies Toward 4-Nitrophenol as an Organic Pollutant
The electrochemical behavior of as-synthesized electrocatalytic systems was investigated by using cyclic voltammetry (CV) in 0.5 M H 2 SO 4 as a supporting electrolyte. Figure 5A shows the CV response of Bi 2 O 3 NPs, MWCNTs, and Bi 2 O 3 @MWCNTs in the presence of 4-NP (4 mM) in 0.5 M H 2 SO 4 at a scan rate of 50 mV/s. It has been observed that Bi 2 O 3 NPs show the smallest reduction peak but MWCNTs shows a slightly higher reduction peak. In the case of Bi 2 O 3 @MWCNTs a significant increase in reduction peak was observed which confirms the synergistic effect of Bi 2 O 3 and MWCNTs, i.e., due to increases in the electron density of Bi and the higher accessible surface of MWCNTs toward 4-NP sensing.
Electrochemical Impedance Spectroscopy (EIS) is an efficient tool which provides insights about the interfacial picture during electron transfer at interfaces (Wu et al., 2018) during the reductive sensing of 4-NP. In EIS studies the diameter of a semicircle is directly proportional to electron transfer resistance which governs the electron transfer kinetics . Figure 5B shows Nyquist plots for Bi 2 O 3 NPs, MWCNTs and Bi 2 O 3 @MWCNTs in 0.5 M H 2 SO 4 . The larger semi-circle observed for Bi 2 O 3 NPs and MWCNTs reflects a higher electron transfer resistance. On the contrary Bi 2 O 3 @MWCNTs (309 ) shows a small semi-circle which reflects a lower electron transfer resistance which results in higher electrocatalytic activity toward 4-NP reduction compared to Bi 2 O 3 NPs (1,000 ) and MWCNTs (400 ). These result are in good agreement with cyclic voltammetry results. Table 1 shows comparative data of the onset potential and peak current density of electrocatalysts, it can be seen that the Bi 2 O 3 @MWCNTs nanocomposite has a lower negative onset potential and higher peak current density as compared to Bi 2 O 3 NPs and MWCNTs, respectively. Figure 6A shows superimposed CV curves for different concentrations of 4-NP on Bi 2 O 3 @MWCNTs in 0.5 M H 2 SO 4 at a scan rate of 50 mV/s. The reduction peak current increases linearly with concentration (1, 2, 4, 6, 8, 10 mM) which means Bi 2 O 3 @MWCNTs shows efficient electrochemical sensing toward 4-NP. Figure 6B shows scan rate dependent studies on the reduction peak current of Bi 2 O 3 @MWCNTs, in 0.5 M H 2 SO 4 at a concentration of 4 mM. The variations in their linearity with current density and positive shift in potential could be due to diffusion controlled 4-NP reduction processes as shown in Figures 6C,D. Figure 7A shows 4-NP electrochemical sensing by the Bi 2 O 3 @MWCNTs nanocomposite in acidic, basic and neutral media. As electroreduction of 4-NP is a proton involving step, the pH of the supporting electrolyte affects the electrochemical processes. From Figure 7A it is evident that a distinct reduction peak is observed in the case of an acidic medium as compared to neutral and basic media. This is due to the fact that protons replaced the 4-NP present on electrode surface, as the pH of the medium is increased nitrogen anions prevent the approach of 4-NP to the electrode surface (Wu et al., 2018). The mechanism has been proposed for the electrochemical reduction of 4-NP on Bi 2 O 3 @MWCNTs on the basis of CV is shown in Figure 7B. There is not an appreciable peak observed in the absence of 4-NP (red line) while in the presence (black line) of 4-NP the redox couple is observed at Ea 1 = 0.52 and Ec 1 = 0.43 and the reduction peak is at Ec 2 = −0.37 which confirms the reduction of 4-NPs.
The C 2 peak symbolizes the formation of 4hydroxylaminophenol which is an irreversible reaction while the redox couple is due to the interconversion of 4hydroxylnitrophenol and 4-nitrosophenol (Rajkumar et al., 2018;He et al., 2019). The electrochemical mechanism is given in Figure 8.

Limit of Detection (4-NP)
The limit of detection is an important parameter for analytical method validation. Figure 6A depicts cyclic voltammetric response for different concentrations of 4-NP vs. peak current densities. Accordingly, Where, S, Standard deviation; M, Slope point. The LOD is calculated by using equation 1 and the LOD was calculated to be 0.1 µM (Mulik et al., 2018). Finally it is found that a smaller LOD confirms the actual apparent concentration of the 4-NP reduction reaction. In addition to this, our work has been compared with the reported literature. Table 2 summarizes reported electrocatalytic systems with their linearity range and LOD upon comparison proposed Bi 2 O 3 @MWCNTs system for 4-NP reduction (Mulik et al., 2018).

CONCLUSION
The Bi 2 O 3 @MWCNTs electrocatalyst was synthesized by using a simple chemical reduction method. As-synthesized nanomaterials MWCNTs, Bi 2 O 3 and Bi 2 O 3 @MWCNTs NPs have been well-characterized by FTIR which confirms the Bi-O bonding in Bi 2 O 3 @MWCNT. XRD shows the Bi 2 O 3 @MWCNTs was in an α-metastable crystal structure. Raman spectra show the I D /I G ratio increases in Bi 2 O 3 @MWCNTs as compared with MWCNTs, and it is confirmed that there are more sp 2 C, TEM analysis confirms the average size is ∼10 nm. The improved monometallic Bi supporting MWCNTs provides a higher surface area which results in a significant increase in the electrocatalytic activity with an onset potential of −0.17 V toward electrochemical 4-NP reduction. As compared to reported systems, this is a cost effective and highly efficient system for the determination of 4-NP.

DATA AVAILABILITY STATEMENT
The datasets generated for this study are available on request to the corresponding author.

AUTHOR CONTRIBUTIONS
RD has conducted all experiments and written the manuscript. AM and BM have helped to interpret the characterization data, whereas BS has invigilated the whole project with his expert advice and fruitful suggestions.